The South Korean and U.S. military yesterday began an annual major joint exercise to test defenses against North Korea, a drill denounced by Pyongyang which vowed to strengthen its nuclear deterrent.

More than 30,000 U.S. troops, including most of those based in the South plus 3,000 from overseas, are taking part in the drill known as Ulchi Freedom Guardian, which ends on Aug. 31, the U.S. forces said in a statement.

No field trainingSeoul’s defense ministry could not say how many South Korean troops were taking part but Yonhap news agency put the number at 56,000. The drill does not involve field training and is largely a computer-simulated exercise, with troops staying at their normal bases. U.S. and South Korean forces insist it is defensive while the North called it a drill for a preemptive nuclear attack. “The prevailing situation requires (North Korea) to bolster up the war deterrent physically and goes to prove that it was entirely just when it determined to fully reexamine the nuclear issue,” the North’s foreign ministry said. The war deterrent “serves as a just means for retaliation,” it said in a statement published by state media.

“This is an all-powerful treasured sword for protecting the sovereignty of the country and a powerful means for deterring the war on the Korean peninsula,” the ministry said. General James Thurman, commander of the 28,500 U.S. troops based in the South, called Ulchi Freedom Guardian “a key exercise in strengthening the readiness of Republic of Korea (South Korean) and U.S. forces.”

On the eve of the exercise, the North’s leader Kim Jong-un visited a frontline artillery unit that carried out the deadly 2010 bombardment of a South Korean island near the disputed western sea border.

Kim praised its personnel as heroes and told them never to tolerate enemy aggression. The two Koreas have remained technically at war since their 1950-53 conflict ended in an armistice, without a subsequent peace treaty.